Resonant transducer, manufacturing method therefor, and multi-layer structure for resonant transducer

ABSTRACT

A resonant transducer includes a silicon single crystal substrate, a silicon single crystal resonator disposed over the silicon single crystal substrate, a shell made of silicon, surrounding the resonator with a gap, and forming a chamber together with the silicon single crystal substrate, an exciting module configured to excite the resonator, a vibration detecting module configured to detect vibration of the resonator, a first layer disposed over the chamber, the first layer having a through-hole, a second layer disposed over the first layer, a third layer covering the first layer and the second layer, and a projection extending from the second layer toward the resonator, the projection being spatially separated from the resonator, the projection being separated from the first layer by a first gap, the second layer being separated from the first layer by a second gap, the first gap is communicated with the second gap.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a resonant transducer, a manufacturingmethod therefor, and a multi-layer structure for a resonant transducer.

Priority is claimed on Japanese Patent Application No. 2013-153874,filed Jul. 24, 2013, the content of which is incorporated herein byreference.

2. Description of Related Art

A resonant transducer has been known as a sensor for detecting physicalstress. For example, the resonant transducer includes a vacuum chamber,a microscopic resonator disposed in the chamber, and a vibrationdetector detecting vibrations of the microscopic resonator. As shown inJapanese Unexamined Patent Application Publication No. 2012-58127, thechamber, the microscopic resonator, and the vibration detector aredisposed in a silicon substrate (silicon wafer).

SUMMARY OF THE INVENTION

A manufacturing method of a resonant transducer may include a siliconsingle crystal substrate, a silicon single crystal resonator disposedover the silicon single crystal substrate, a shell made of silicon,surrounding the resonator with a gap, and forming a chamber togetherwith the silicon single crystal substrate, an exciting module configuredto excite the resonator, a vibration detecting module configured todetect vibration of the resonator, a first layer disposed over thechamber, the first layer having a through-hole, a second layer disposedover the first layer, a third layer covering the first layer and thesecond layer, and a projection extending from the second layer towardthe resonator, the projection being spatially separated from theresonator, the projection being separated from the first layer by afirst gap, the second layer being separated from the first layer by asecond gap, the first gap is communicated with the second gap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating the resonant transducer of afirst embodiment.

FIG. 2 is a plane view illustrating the resonant transducer of the firstembodiment.

FIG. 3 is a zoomed sectional view around a main part of a resonator anda shell.

FIG. 4 is a zoomed diagrammatic perspective view illustrating a shape ofa through-hole.

FIG. 5 is a circuit diagram illustrating the resonant transducer.

FIG. 6 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thefirst embodiment.

FIG. 7 is a sectional view of the resonant transducer for describing amanufacturing method of the resonant transducer of the first embodiment.

FIG. 8 is a sectional view of the resonant transducer for describing amanufacturing method of the resonant transducer of the first embodiment.

FIG. 9 is a sectional view of the resonant transducer for describing amanufacturing method of the resonant transducer of the first embodiment.

FIG. 10 is a sectional view of the resonant transducer for describing amanufacturing method of the resonant transducer of the first embodiment.

FIG. 11 is a sectional view of the resonant transducer for describing amanufacturing method of the resonant transducer of the first embodiment.

FIG. 12 is a sectional view of the resonant transducer for describing amanufacturing method of the resonant transducer of the first embodiment.

FIG. 13 is a sectional view of the resonant transducer for describing amanufacturing method of the resonant transducer of the first embodiment.

FIG. 14 is a sectional view of the resonant transducer for describing amanufacturing method of the resonant transducer of the first embodiment.

FIG. 15 is a sectional view of the resonant transducer for describing amanufacturing method of the resonant transducer of the first embodiment.

FIG. 16 is a zoomed diagrammatic perspective view illustrating a mainpart of the resonant transducer of the second embodiment.

FIG. 17 is a zoomed diagrammatic perspective view illustrating a mainpart of the resonant transducer of the third embodiment.

FIG. 18 is a zoomed diagrammatic perspective view illustrating a mainpart of the resonant transducer of the fourth embodiment.

FIG. 19 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thesecond embodiment.

FIG. 20 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thesecond embodiment.

FIG. 21 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thesecond embodiment.

FIG. 22 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thesecond embodiment.

FIG. 23 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thesecond embodiment.

FIG. 24 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thethird embodiment.

FIG. 25 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thethird embodiment.

FIG. 26 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thethird embodiment.

FIG. 27 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thethird embodiment.

FIG. 28 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thethird embodiment.

FIG. 29 is a sectional view illustrating the resonant transducer fordescribing a manufacturing method of the resonant transducer of thethird embodiment.

FIG. 30 is an exemplary sectional view illustrating a main part of theresonant transducer in the related art.

FIG. 31 is a drawing for describing a condition of discharging theetching liquid in the manufacturing method of the resonant transducer inthe related art.

DETAILED DESCRIPTION OF THE INVENTION

Before describing some embodiments, the related art will be explainedwith reference to one or more drawings, in order to facilitate theunderstanding of the embodiments.

FIG. 30 is a drawing showing an example of a resonant transducer 100 inthe related art. The resonant transducer 100 shown in FIG. 30 includes asilicon substrate 101 for a measurement diaphragm. The resonanttransducer 100 also includes a multi-layer structure 110 over thesubstrate 101. The multi-layer structure 110 includes oxidized layer 113and insulated layer 114. The multi-layer structure 110 also includesfirst electrode 111 and second electrode 112, which are above theoxidized layer 113 and below the insulated layer 114. The firstelectrode 111 and the second electrode 112 are separated by a chamber102 in which a resonator 103 is disposed. The resonator 103 is separatedby gaps from the first and second electrodes 111 and 112. A shell 104 isdisposed on the insulated layer 114 and over the resonator 103, so thatthe shell 104 seals the chamber 102.

The shell 104 includes a first polysilicon layer (first layer) 105, asecond polysilicon layer (second layer) 106, and a third polysiliconlayer (third layer) 107. The first polysilicon layer 105 is disposedover the chamber 102. The second polysilicon layer 106 is disposed overthe first polysilicon layer 105. The second polysilicon layer 106 has alayer and a plug. The layer extends over the first polysilicon layer105. The plug is in the first polysilicon layer 105.

For example, a through-hole 108 in the first polysilicon layer 105 is aflow path in which etching liquid flows in a process of forming thechamber 102. After the through-hole 108 is used as a flow path in whichetching waste liquid flows, the through-hole 108 is filled up with theplug of the second polysilicon layer 106 with no space in thethrough-hole 108. The resonant transducer 100 measures stress(deformation) applied to the resonator 103 by detecting a change ofresonant frequency of the resonator 103.

The resonant transducer 100 described above uses liquid in a process ofdischarging the etching waste liquid and a process of washing after thedischarge process. As shown in FIG. 31, if a droplet Q of the liquidremains between the resonator 103 and the first polysilicon layer 105including the through-hole 108, the resonator 103 is drawn to a side 105f of the first polysilicon layer 105 by meniscus force of the liquid. Asthe result, the resonator 103 adheres to the first polysilicon layer105. Therefore, it remains possible that the resonant transducer 100cannot detect the stress.

Also, in a process of forming plug with no space in the through-hole108, the bigger a diameter of the through-hole 108 is, the thicker thesecond polysilicon layer 106 accumulated in the chamber 102 is. Thesecond polysilicon layer 106 accumulated in an interspace between theresonator 103 and the first and second electrodes 111 and 112 lying ateach side of the resonator 103 causes variation of magnitude of anoutput signal. Also, it remains possible that the first and secondelectrodes 111 and 112 are shorted by the second polysilicon layer 106accumulated in the chamber 102, and the resonant transducer 100 cannotoutput the output signal.

Some embodiments of the present invention will be now described hereinwith reference to illustrative preferred embodiments. Those skilled inthe art will recognize that many alternative preferred embodiments canbe accomplished using the teaching of the present invention and that thepresent invention is not limited to the preferred embodimentsillustrated herein for explanatory purposes.

A resonant transducer and a manufacturing method therefor according toembodiments of the present invention will be described below in detail,with references to the drawings. The present embodiment is described indetail in order to make the scope of the invention easier to understand,and the present embodiment does not limit the present invention inasmuchas there are no particular specifications. Some of the drawings used inthe following description show enlarged views of significant portionsfor the sake of convenience in order to make the characteristics of thepresent invention easier to understand, and the dimensional ratios andother features of the constituent elements are not meant to be limitedto those presented herein.

A Resonant Transducer First Embodiment

FIG. 1 is a sectional view illustrating the resonant transducer 10 of afirst embodiment along a thickness direction. FIG. 2 is a plane viewillustrating the resonant transducer 10 of the first embodiment withouta shell. The resonant transducer 10 of an exemplary embodiment of thepresent invention includes a resonator 12 formed on a substrate 11 madeof silicon single crystal. Interspaces are disposed around theresonator. Also, the resonant transducer 10 includes a shell 14surrounding the resonator 12 and a chamber 21 with the substrate 11. Theshell 14 is a multi-layer structure for the resonant transducer 10.

The chamber 21 is disposed on a side 11 a of the substrate 11. Theresonator 12, a first electrode 15, a second electrode 16, and a thirdelectrode 17 are disposed in the chamber 21. Also, epitaxial layer 18 isdisposed out of the chamber 21. The epitaxial layer 18, the resonator12, the first electrode 15, the second electrode 16, and the thirdelectrode 17 are made of same material such as a boron-doped lowresistance P-type semiconductor.

The resonator 12 is integral with the first electrode 15 andelectrically connected to the first electrode 15. When viewed from theshell 14, the resonator 12 is substantially narrow plate-like structure.A length of the resonator 12 in the thickness direction is longer than awidth of the resonator 12 in a planar direction of the substrate 11.Also, predetermined tensile stress against the substrate 11 is added tothe plate-like resonator 12. One end of the resonator 12 is integrallyconnected to the first electrode 15.

The second electrode 16 and the third electrode 17 are substantiallyrectangular-shaped electrodes. The second electrode 16 and the thirdelectrode 17 are disposed in both sides of the resonator 12 in alongitudinal direction keeping predetermined gap from the resonator 12.Connection points 15 a, 16 a, and 17 a to be connected to an externalelectrical circuit are respectively disposed on the first electrode 15,the second electrode 16, and the third electrode 17. For example, theconnection points 15 a, 16 a, and 17 a are made of metal.

Although insulated layer 22 is disposed between each of the electrodes15 to 17 and the shell 14, the insulated layer 22 is not disposed in thechamber 21. Also, although insulated layer 23 is disposed between eachof the electrodes 15 to 17 and the substrate 11, the insulated layer 23is not disposed in the chamber 21. In a manufacturing process of theresonant transducer, the insulated layer 23 is formed by using a SOIsubstrate as the substrate 11. The manufacturing process of the resonanttransducer will be described in detail.

Inside of the chamber 21 is kept in a predetermined degree of vacuum.For example, pressure in the chamber 21 is less than or equal to severaltens of Pa so that measurement accuracy of the resonant frequency can beimproved by suppressing energy loss of the resonator in a resonantstate. The resonator 12, the first electrode 15, the second electrode16, and the third electrode 17 are disposed leaving predetermined gapfrom members surrounding the chamber 21. The members surrounding thechamber 21 are the substrate 11, the epitaxial layer 18, and the shell14.

FIG. 3 is a zoomed sectional view around the resonator and the shell.The shell 14 includes a first polysilicon layer (first layer) 26, asecond polysilicon layer (second layer) 27, and a third polysiliconlayer (third layer) 28. The first polysilicon layer 26 is disposed overthe insulated layer 22. The second polysilicon layer 27 is disposed overthe first polysilicon layer 26. The third polysilicon layer 28 coversthe first polysilicon layer 26 and the second polysilicon layer 27.

In the present embodiment, the three layers 26 to 28 (the first layer,the second layer, and the third layer) of the shell 14 may be made ofpolysilicon. However, the present embodiment is not limited topolysilicon. For example, the three layers 26 to 28 may be made of anyone of amorphous silicon, SiC, SiGe, Ge, and so on.

The first polysilicon layer (first layer) 26 is in contact with theinsulated layer 22. Also, the first polysilicon layer is disposed abovethe chamber. A through-hole 25 is disposed at an overlapped part of thefirst polysilicon layer 26 and the chamber 21. The through-hole 25extends along a thickness direction of the first polysilicon layer 26.In following description, an opening portion of the through-hole 25 at aside of the chamber 21 may be called a first opening portion 25 a. Also,an opening portion of the through-hole 25 at a side of the secondpolysilicon layer 27 may be called a second opening portion 25 b.

In the present embodiment, as shown in FIG. 4, when viewed from thethird polysilicon layer 28, the first opening portion 25 a and thesecond opening portion 25 b of the through-hole 25 arerectangular-shaped sections extending along the resonator 12. Morespecifically, the through-hole 25 is a cuboid-shaped narrow spaceextending along a longitudinal direction of the resonator 12.

The second polysilicon layer 27 is disposed near the second openingportion 25 b of the through-hole 25, and covers around the secondopening portion 25 b. Specifically, the second polysilicon layer 27extends along a longitudinal direction of the through-hole 25 around thesecond opening portion 25 b in predetermined width.

Also, the second polysilicon layer 27 has a projection. The projection29 is integral with the second polysilicon layer 27 at a side of thethrough-hole 25 of the second polysilicon layer 27. The projection 29enters from the second opening portion 25 b into the through-hole 25. Anend face 29 a of the projection 29 is disposed in a position father awayfrom the resonator 12 than a face 26 a of the first polysilicon layer26.

The third polysilicon layer 28 covers the first polysilicon layer 26 andthe second polysilicon layer 27. Specifically, the third polysiliconlayer 28 is in contact with the second polysilicon layer 27 in an areawhere the second polysilicon layer 27 exists. Also, the thirdpolysilicon layer 28 is in contact with the first polysilicon layer 26in outside of the second polysilicon layer 27.

In the shell 14 having the foregoing multi-layer structure, a gap 31exists between the first polysilicon layer 26 and the second polysiliconlayer 27. The gap 31 extends from a first gap E1 between the firstpolysilicon layer 26 and the projection 29 to a second gap E2 betweenthe first polysilicon layer 26 and the second polysilicon layer 27. Thefirst gap E1 is communicated with the second gap E2.

Specifically, in the sectional view shown in FIG. 1, the gap 31 issubstantially L-shaped narrow space between the first polysilicon layer26 and the second polysilicon layer 27. The first gap E1 is one open endof the gap 31. The first gap E1 exists between a side wall of the firstopening portion 25 a of the through-hole 25 and the projection 29 of thesecond polysilicon layer 27. Also, the second gap E2 is other open endof the gap 31. The second gap E2 is a gap between the first polysiliconlayer 26 and the second polysilicon layer 27.

For example, the gap 31, that is to say, a distance between the firstpolysilicon layer 26 and the second polysilicon layer 27 may be adistance where etching liquid used in a process of forming the chamber21 can flow in and flow out.

Non-empty spacers 32 are disposed on the second polysilicon layer 27.The spacers 32 are integral with the second polysilicon layer 27. Thespacers 32 project from the second polysilicon layer 27. End faces ofthe spacers 32 come into contact with the first polysilicon layer 26.The spacers 32 form the second gap E2 to prevent the gap 31 from beingnarrowed by stress. A height of the spacer 32 is substantially equal tothe gap 31.

As shown in FIG. 4, the spacers 32 are disposed along the longitudinaldirection of the through-hole 25. Liquid flowing in the gap 31 (shown inthe FIG. 3) such as the etching liquid flows between the spacers 32. Forthis reason, the spacers 32 form the second gap E2 of the gap 31, andprevent interrupting the flow of the liquid. In the present embodiment,each of the spacers 32 is long cylindroid-shaped.

FIG. 5 is a circuit diagram of the resonant transducer. The resonanttransducer 10 includes an exciting module 41 for exciting the resonator12 and a vibration detecting module 42 for detecting vibration of theresonator 12. The exciting module 41 includes the second electrode 16and a drive electrical source 43. The vibration detecting module 42includes the first electrode 15, the third electrode 17, a biaselectrical source 44, resistors R1, R2, and R3, operational amplifiersOP1 and OP2, and so on.

The drive electrical source 43 applies alternating-current voltage ofpredetermined drive voltage Vi. The bias electrical source 44 appliesdirect-current voltage of predetermined bias voltage Vb. The firstelectrode 15 is applied the constant bias voltage Vb from the biaselectrical source 44. The second electrode 16 is applied the alternatingdrive voltage Vi from the drive electrical source 43. Detection signalsaccording to vibrational frequency of the resonator 12 is output fromthe third electrode 17.

Operations of the resonant transducer are described below. After theconstant bias voltage Vb is applied to the first electrode 15 and thealternating drive voltage Vi is applied to the second electrode 16,electrostatic suction power is generated between the resonator 12connected to the first electrode 15 and the second electrode 16. At thetime, the resonator 12 vibrates (resonates) at constant resonantfrequency.

On the other hand, electrical charge is generated between the resonator12 connected to the first electrode 15 and the third electrode 17 by thebias voltage Vb applied to the first electrode 15. When electrostaticcapacity between the resonator 12 and the third electrode 17 is changedin accordance with vibration of the resonator 12, a detection signalaccording to the change of the electrostatic capacity is generated. Thedetection signal is alternating current. The operational amplifiers OP1and OP2 amplify the detection signal. A counter reads the detectionsignal amplified by the operational amplifiers OP1 and OP2 as a voltagechange so that the vibrational frequency of the resonator 12 can bemeasured.

When the resonator 12 is stressed by the stress, the vibrationalfrequency of the resonator 12 is changed in accordance with an amount ofa strain of the resonator 12. An amount of the strain of the resonator12, that is to say, the stress applied to the resonator 12 can bemeasured.

In the constitution, because it is possible to separate the secondelectrode 16 as an exciting electrode from the third electrode 17 as adetecting electrode, parasitic capacity between the second electrode 16and the third electrode 17 decreases. As a result, cross talk of thedrive voltage Vi on the detection circuit can be suppressed. Also,signal-to-noise ratio can be improved.

A Manufacturing Method of the Resonant Transducer First Embodiment

A manufacturing method of the resonant transducer and an operation ofthe resonant transducer are described below.

FIG. 6 to FIG. 15 are zoomed sectional views of a main part of theresonant transducer for describing the manufacturing method of theresonant transducer in stages. Also, FIG. 6 to FIG. 15 are sectionalviews along a line A-A in FIG. 2.

First, as shown in FIG. 6, a SOI substrate 51 where an oxidized layer 52and a superficial silicon layer 53 are formed on the substrate 11 isprepared. For example, a thickness of the oxidized layer 52 is about 2micrometers. Also, a thickness of the superficial silicon layer 53 as anactive layer is about 1 micrometer.

Next, as shown in FIG. 7, an epitaxial silicon layer 54 including a highlevel of boron is formed on the superficial silicon layer 53 as theactive layer by epitaxial growth. The epitaxial silicon layer 54including a high level of boron is low electrical resistance and behaveslike a conductor. In post-process, the resonator 12, the first electrode15, the second electrode 16, and the third electrode 17 (shown in FIG.2) are formed in the epitaxial silicon layer 54.

Also, as the epitaxial silicon layer 54 including a high level of boronhas grater tension stress than the substrate 11, the epitaxial siliconlayer 54 generates tension to the resonator 12 formed in thepost-process. When the stress is applied to the resonator 12 in atension condition, the stress is proportional to a square of frequency,an extremely linear characteristic is obtained. On the other hand, as anoperation in a compression stress condition has a non-linearcharacteristic, an operation of the resonant transducer 10 is performedin a tension stress condition.

A growth condition of the epitaxial silicon layer 54 including a highlevel of boron is (a) to (d) described below.

(a) growth temperature is 1030 degrees Celsius,(b) in H₂ gas,(c) dichlorosilane (SiH₂Cl₂) is used as ingredient gas of silicon, and(d) diborane (B₂H₆) is used as ingredient gas of boron which is animpurity.

Also, the epitaxial silicon layer 54 including a high level of boron isgrown to, for example, about 9 micrometers by performing the epitaxialgrowth for predetermined time. Then a sum of the thickness of theepitaxial silicon layer 54 and the thickness of the superficial siliconlayer 53 is about 10 micrometers.

Next, as shown in FIG. 8, patterning of the epitaxial silicon layer 54including a high level of boron is performed. Trenches T to becomeoutline forms of the resonator 12, the first electrode 15 (shown in FIG.2), the second electrode 16, and the third electrode 17 are formed onthe epitaxial silicon layer 54. For example, the patterning of theepitaxial silicon layer 54 is performed by applying resist material.Also, the patterning is performed by a stepper apparatus.

For example, the stepper apparatus has resolution about 0.3 micrometers.Also, the stepper is capable of exposing submicron lines and spaces. Theoutline form pattern of the resonator 12, the first electrode 15, thesecond electrode 16, and the third electrode 17 are formed by thestepper apparatus.

The resist layer formed by the stepper apparatus is used as a mask, andthe epitaxial silicon layer 54 is etched. The trenches T shaping theoutline forms of the resonator 12, the first electrode 15, the secondelectrode 16, and the third electrode 17 are formed. For example, theepitaxial silicon layer 54 is etched by dry etching. The dry etching isperformed until an etching position reaches the oxidized layer 52 on thesubstrate 11. The resonator 12, the first electrode 15, the secondelectrode 16, and the third electrode 17 are electrically separated eachother.

In forming the trenches T by the dry etching, it is suitable thatconcave-convex portions are formed on a side wall of the trenches T byrepeatedly performing a silicon etching process and a deposition processof a CF polymer. For example, stripes of which a width of theconcave-convex portions is about 0.1 micrometer or more and a pitch ofthe concave-convex portions is about 0.1 to 1 micrometer are formed byadjusting an etching time and a deposition time.

Next, as shown in FIG. 9, an insulated layer 56 is formed over theepitaxial silicon layer 54. Layered structure 59 includes the substrate11, the epitaxial silicon layer 54, the oxidized layer 52, and theinsulated layer 56. The trenches T shaping the outline forms of theresonator 12, the first electrode 15, the second electrode 16, and thethird electrode 17 are filled with the insulated layer 56. The insulatedlayer 56 accumulates on the epitaxial silicon layer 54 by apredetermined thickness. For example, the insulated layer 56 is made ofoxidized silicon. In forming the insulated layer 56, for example, openend portions of the trenches T are filled with a LP-CVD oxidized film ora plasma-CVD oxidized film of tetraethoxysilane (TEOS).

For example, the LP-CVD oxidized film is formed in a low pressurecondition of 700 degrees Celsius and 50 Pascal by bubbling a TEOS tank,introducing nitrogen gas and oxygen gas, pyrolyzing the TEOS, andfilling the trenches T with oxidized silicon.

The plasma CVD oxidized film is formed by a process of generating plasmaby introducing TEOS and oxygen gas in vacuum, filling the trenches Twith oxidized silicon on a substrate put on a stage heated to 400degrees Celsius. As step coverage of the plasma CVD oxidized film isinferior in quality, the film is not easily formed in a deepest part ofthe trenches T and voids V are formed in a part of the insulated layer56.

Next, as shown in FIG. 10, for example, the first polysilicon layer(first layer) 26 of which a thickness is several micrometers is formedover the insulated layer 56 covering the epitaxial silicon layer 54. Thefirst polysilicon layer 26 is a part of the shell 14 covering thechamber 21 formed in the post-process.

Next, as shown in FIG. 11, the through-hole 25 is formed in a part ofthe first polysilicon layer 26. For example, the through-hole 25 isformed in a position facing the resonator 12. Also, for example, afterforming the resist layer for shaping an outline form of the firstpolysilicon layer 26, the through-hole 25 passing through the firstpolysilicon layer 26 in a thickness direction is formed by the dryetching. For example, the through-hole 25 is a cuboid-shaped narrowspace extending along the longitudinal direction of the resonator 12(shown in FIG. 4).

Next, as shown in FIG. 12, an oxide layer (sacrifice layer) 57 isformed. The oxide layer 57 covers the first polysilicon layer 26 and aninner surface of the through-hole 25. Dimples 58 for forming outershapes of the spacers 32 of the second polysilicon layer 27 are formedaround the second opening portion 25 b of the through-hole 25. Thespacers 32 will be formed in the post-process.

For example, a LP-CVD apparatus forms the oxide layer 57 of whichthickness is about 100 nanometers and the dimples 58 are formed by usingresist material removing only an area of the dimples 58 with bufferedhydrofluoric acid.

Next, as shown in FIG. 13, the second polysilicon layer (second layer)27 is formed to cover the through-hole 25 and an area surrounding thethrough-hole 25. The projection 29 is integral with the secondpolysilicon layer 27. Also, the spacers 32 are integral with the secondpolysilicon layer 27. The projection 29 enters into the through-hole 25.The dimples 58 shape the spacers 32.

Next, as shown in FIG. 14, a whole of the oxide layer (sacrifice layer)57 (shown in FIG. 13), the insulated layer 56 around the resonator 12(shown in FIG. 13), and the oxidized layer 52 around the resonator 12(shown in FIG. 13) are removed by etching with dilute HF solution. Bythe process, the chamber 21 is formed around the resonator 12, and thegap around the resonator 12 is kept.

On the other hand, by removing the oxide layer 57 formed between thefirst polysilicon layer 26 and the second polysilicon layer 27, the gap31 is formed between the first polysilicon layer 26 and the secondpolysilicon layer 27. The gap 31 extends from a first gap E1 between thefirst polysilicon layer 26 and the projection 29 to a second gap E2between the first polysilicon layer 26 and the second polysilicon layer27. The first gap E1 is communicated with the second gap E2. The diluteHF solution reaches the insulated layer 56 around the resonator 12 andthe oxidized layer 52 via the gap 31.

Waste solution of the dilute HF solution used for forming the chamber 21by etching the insulated layer 56 and the oxidized layer 52 around theresonator 12 is discharged from inside to outside of the chamber 21 viathe gap 31. When almost all of the waste solution is discharged and alittle waste solution remains between the resonator 12 and the end face29 a of the projection 29 of the second polysilicon layer 27, meniscusforce may be applied to the waste solution. If the meniscus force isapplied to the waste solution, there is a worry that the resonator 12bends toward the projection 29 of the second polysilicon layer 27 andthe resonator 12 becomes deformed.

However, as the narrow gap 31 is formed between the first polysiliconlayer 26 and the second polysilicon layer 27, a little waste solutionaround the first opening portion 25 a is soaked up quickly by capillaryaction. By this process, around the first opening portion 25 a of thethrough-hole 25, the projection 29 projects toward the chamber 21 so asto prevent the resonator 12 from being deformed and fixed. By decreasinga contact area of the end face 29 a of the second polysilicon layer 27and the resonator 12, in a process of removing the etching wastesolution and water droplet of a washing process, it is preventable thatthe resonator 12 bends and adheres to an under surface of the firstpolysilicon layer 26. Also, the gap 31 between the first polysiliconlayer 26 and the second polysilicon layer 27 prevents a variability ofintensity of output signal, a variability of resonant frequency of theresonator 12, and output failure according to a short of electrodes.

After that, as shown in FIG. 15, the third polysilicon layer (thirdlayer) 28 is formed. The third layer 28 covers the first polysiliconlayer 26 and the second polysilicon layer 27 for vacuum sealing. Thethird polysilicon layer 28 seals the gap 31 at a position of the secondgap E2. The vacuum sealing by the third polysilicon layer 28 isperformed in a condition that stretching strain is generated in thethird polysilicon layer 28 or remaining compression strain is verylittle. For example, the vacuum sealing is performed by reduced pressureepitaxial apparatus at equal to or less than 900 degrees Celsius. SiH₄or mixture of SiH₄ and hydrogen can be used as ingredient gas.

After that, holes exposing the connection points 15 a, 16 a, and 17 a(shown in FIGS. 1 and 2) are formed at a position where the firstpolysilicon layer 26 is in contact with the third polysilicon layer 28.By these process, a sensor portion of the vibration transducer 10 can beformed.

As described above, in the resonant transducer, the manufacturing methodtherefor, and a multi-layer structure for a resonant transducer of thepresent embodiment, the contact area of the end face 29 a and theresonator 12 is decreased. Also, it is preventable that the resonator 12bends and adheres to the first polysilicon layer 26. Therefore, even ifthe meniscus force is applied to the droplet remaining between theresonator 12 and the first polysilicon layer 26, it is preventable thatthe resonator 12 adheres to the first polysilicon layer.

Also, as almost all the polysilicon layer in the chamber 21 can berestrained by the narrow gap 31 between the first polysilicon layer 26and the second polysilicon layer 27, a variability of intensity ofoutput signal, a variability of resonant frequency of the resonator 12,and output failure according to a short of electrodes are preventable.

Other embodiments of the resonant transducer will be described below. Ineach of the embodiments, same components as the first embodiment arenumbered in the same manner as the first embodiment, and the explanationof the components are left out.

A Resonant Transducer Second Embodiment

In the first embodiment, the through-hole 25 in the first polysiliconlayer 26 is a cuboid-shaped narrow space extending along thelongitudinal direction of the resonator 12. However, the shape of thethrough-hole 25 and the shape of the second polysilicon layer 27covering the through-hole 25 are not limited thereto.

In the resonant transducer 60 shown in FIG. 16, cylindrical-shapedthrough-holes 61 are disposed along the longitudinal direction of theresonator 12 on the first polysilicon layer (first layer) 26. Whenviewed from above, second opening portions 61 b of the through-holes 61are circular. The through-holes 61 are disposed along the longitudinaldirection of the resonator 12. Projecting portions 62 are integral withthe second polysilicon layer (second layer) 27. The projections 62 enterinto the through-holes 61 respectively. A part of the gap 31 iscylindrical between an outer surface of the projection 62 and an innersurface of the through-hole 61.

Non-empty spacers 63 are integral with the second polysilicon layer 27.The spacers are in contact with a plane around second opening portions61 b of the through-holes 61 in the first polysilicon layer 26. Forexample, in the resonant transducer 60 shown in FIG. 16, the spacers 63come into contact with the first polysilicon layer 26 at positionssurrounding the second opening portions 61 b of the through-holes 61.

A Resonant Transducer Third Embodiment

In the resonant transducer 65 shown in FIG. 17, cuboid-shapedthrough-holes 66 are disposed along the longitudinal direction of theresonator 12 on the first polysilicon layer (first layer) 26. Whenviewed from above, second opening portions 66 b of the through-holes 66are rectangular. The through-holes 66 are disposed along thelongitudinal direction of the resonator 12. Projecting portions 67 areintegral with the second polysilicon layer (second layer) 27. Theprojections 67 enter into the through-holes 66 respectively. A part ofthe gap 31 is rectangular-section cylindrical between an outer surfaceof the projection 67 and an inner surface of the through-hole 66.

Non-empty spacers 68 are integral with the second polysilicon layer 27.The spacers 68 touch the first polysilicon layer 26 around secondopening portions 66 b of the through-holes 66. For example, in theresonant transducer 65 shown in FIG. 17, the spacers 68 come intocontact with the first polysilicon layer 26 at both sides along alongitudinal direction of the second opening portions 66 b of thethrough-holes 66.

The shapes of the through-holes 66 may be oval-shapes, ellipse-shapes,triangular-shapes, polygonal-shapes, indefinite-shapes, or the like. Theshapes of the through-holes 66 are not limited thereto.

The shapes of the spacers forming the gap 31 (shown in FIG. 3) betweenthe first polysilicon layer 26 and the second polysilicon layer 27 arenot limited to the shapes shown in the first embodiment or the secondembodiment. For example, the spacers may be irregular concavities andconvexities formed on at least one of the first polysilicon layer 26 andthe second polysilicon layer 27 to allow passage of fluid. In this case,the spacers may be rough surfaces on at least one of the firstpolysilicon layer 26 and the second polysilicon layer 27.

A Resonant Transducer Fourth Embodiment

FIG. 18 is a zoomed sectional view illustrating a resonant transducer ofthe fourth embodiment. In the transducer 70 of the present embodiment,the first polysilicon layer (first layer) 26 of the shell 14 has thethrough-hole 25 extending toward the chamber 21. The resonator 12 isdisposed in the chamber 21 and vibrates. A projection 71 is integralwith the second polysilicon layer (second layer) 27. The projection 71enters into the through-hole 25. The gap 31 exists between the outersurface of the projection 71 and the inner surface of the through-holes25.

An end face 71 a of the projection 71 is disposed immediately above theresonator 12. A distance between the end face 71 a and the resonator 12is shorter than a distance between a face 26 f of the first polysiliconlayer 26 and the resonator 12. Therefore, the projection 71 extends fromthe second polysilicon layer 27 to the first opening portion 25 a, andprojects into the chamber 21.

As the projection 71 projects into the chamber 21, it is possible toprevent the resonator 12 from coming into contact with the firstpolysilicon layer 26 more surely. For example, in a process of formingthe chamber 21 of the resonant transducer 70, when discharging theetching waste liquid through the gap 31, the meniscus force may beapplied to the waste liquid remaining between the resonator 12 and thefirst opening portion 25 a of the first polysilicon layer 26. Even ifthe resonator 12 bends widely toward the first polysilicon layer 26 bythe meniscus force, the resonator 12 comes into contact with the endface 71 a of the projection 71 projecting into the chamber 21.

Therefore, it is preventable that the resonator 12 bends widely andcomes into contact with the face 26 f of the first polysilicon layer 26.Specifically, as the end face 71 a of the projection 71 is narrower thanthe resonator 12, even if the resonator 12 bends and comes into contactwith the end face 71 a of the projection 71, a contact area of theresonator 12 and the end face 71 a is small. Therefore, it ispreventable that the resonator 12 adheres to the end face 71 a.

A Manufacturing Method of the Resonant Transducer Second Embodiment

FIG. 19 to FIG. 23 are zoomed sectional views of a main part of theresonant transducer for describing the manufacturing method of theresonant transducer in stages in a second embodiment. FIG. 19 is adrawing for describing a process corresponding to the FIG. 10 in themanufacturing method of the resonant transducer of the first embodiment.As shown in FIG. 19, for example, a first polysilicon layer (firstlayer) 81 of which a thickness is several micrometers is formed on theinsulated layer 56. The layered structure 59 includes the substrate 11,the epitaxial silicon layer 54, the oxidized layer 52, and the insulatedlayer 56. For example, the first polysilicon layer 81 is formed by anepitaxial apparatus. Microscopic concavities and convexities 82according to a grain size are formed on an upper surface of the firstpolysilicon layer 81.

Next, as shown in FIG. 20, the through-hole 83 is formed in a part ofthe first polysilicon layer 81. For example, the through-hole 83 isformed immediately above the resonator 12. Also, for example, afterforming the resist layer for shaping an outline form of the firstpolysilicon layer 81, the through-hole 83 is formed by the dry etching.The through-hole 83 passes through the first polysilicon layer 81 in athickness direction. At this time, microscopic concavities andconvexities 84 are formed on a bottom face of the through-hole 83.

The process shown in FIG. 19 and the process shown in FIG. 20 can beperformed, for example, as a sequence of etching processes of dryetching in which an etching ratio of a polysilicon and an insulationlayer (oxide silicon) is low.

Next, as shown in FIG. 21, an oxide layer 86 is formed. The oxide layer86 covers the microscopic concavities and convexities 82 formed on thefirst polysilicon layer 81, an inner surface of the through-hole 83, andthe microscopic concavities and convexities 84 formed on the bottom faceof the through-hole 83.

Next, as shown in FIG. 22, the second polysilicon layer (second layer)87 is formed to cover the through-hole 83 and an area surrounding thethrough-hole 83. A projection 88 is formed on the bottom surface of thesecond polysilicon layer 87. The projection 88 enters into thethrough-hole 83 covered with the oxide layer 86, and extends to theinsulated layer 56. Concavities and convexities 89 are formed on themicroscopic concavities and convexities 84. Specifically, theconcavities and convexities 89 are formed on an end face 88 a of theprojection 88.

Next, as shown in FIG. 23, a whole of the oxide layer 86, the insulatedlayer 56 around the resonator 12, and the oxidized layer 52 around theresonator 12 are removed by etching with dilute HF solution. By theprocess, the chamber 21 is formed around the resonator 12. The gaparound the resonator 12 is kept.

On the other hand, by removing the oxide layer 86 formed between thefirst polysilicon layer 81 and the second polysilicon layer 87, the gap31 is formed between the first polysilicon layer 81 and the secondpolysilicon layer 87. The gap 31 extends from a first gap E1 between thefirst polysilicon layer 81 and the projection 88 to a second gap E2between the first polysilicon layer 81 and the second polysilicon layer87. The first gap E1 is communicated with the second gap E2. The diluteHF solution flows into the chamber 21 and flows from the chamber 21 viathe gap 31.

By the manufacturing method of the resonant transducer in the presentembodiment, even if a little waste solution remains between theresonator 12 and the first polysilicon layer 81 and the meniscus forceof the waste solution bends the resonator 12 widely toward the firstpolysilicon layer 81, the resonator 12 comes into contact with the endface 88 a of the projection 88 projecting into the chamber 21.Therefore, it is preventable that the resonator 12 adheres to the firstpolysilicon layer 81.

Further, in the manufacturing method of the second embodiment, as themicroscopic concavities and convexities 89 are formed on the end face 88a of the projection 88, a contact area of the projection 88 and theresonator 12 is very small. Therefore, as the resonator 12 does notadhere to the end face 88 a of the projection 88, it is preventable thatthe resonator 12 becomes deformed.

A Manufacturing Method of the Resonant Transducer Third Embodiment

FIG. 24 to FIG. 29 are zoomed sectional views of a main part of theresonant transducer for describing the manufacturing method of theresonant transducer in stages in a third embodiment. FIG. 24 is adrawing for describing a process corresponding to the FIG. 10 in themanufacturing method of the resonant transducer of the first embodiment.As shown in FIG. 24, for example, a first polysilicon layer (firstlayer) 91 of which a thickness is several micrometers is formed on theinsulated layer 56. The layered structure 59 includes the substrate 11,the epitaxial silicon layer 54, the oxidized layer 52, and the insulatedlayer 56. Then, microscopic concavities and convexities 92 are formed onan upper surface of the first polysilicon layer 91 by the dry etching.

Next, as shown in FIG. 25, the through-hole 93 passing through the firstpolysilicon layer 91 is formed. For example, the through-hole 93 isformed immediately above the resonator 12. Also, for example, afterforming the resist layer for shaping an outline form of an openingportion of the through-hole 93 on the first polysilicon layer 91, thethrough-hole 93 is formed by the dry etching. The through-hole 93 passesthrough the first polysilicon layer 91 in a thickness direction.

Further, as shown in FIG. 26, by isotropically etching the insulatedlayer 56 from the through-hole 93, a bottom face 99 is formed in theinsulated layer 56. A diameter of the bottom face 99 is longer than awidth of the through-hole 93. A cross-section shape of the bottom face99 is semicircle. For example, as the characteristic of the etchingliquid used for the etching, an etching rate of oxide silicon is higherthan an etching rate of polysilicon. The etching liquid can be used forisotropically etching.

Next, as shown in FIG. 27, an oxide layer 96 is formed. The oxide layer96 covers the microscopic concavities and convexities 92 formed on thefirst polysilicon layer 91, an inner surface of the through-hole 93, andthe bottom face 99 of the insulated layer 56.

Next, as shown in FIG. 28, the second polysilicon layer 97 is formed.The second polysilicon layer 97 covers the through-hole 93 and an areasurrounding the through-hole 93. A projection 98 is formed on the bottomsurface of the second polysilicon layer 97. The projection 98 entersinto the through-hole 93 covered with the oxide layer 96. The projection98 has an end face 98 a of which shape depends on a shape of the bottomface 99. A cross-section shape of the bottom face 99 is semicircle.

Next, as shown in FIG. 29, a whole of the oxide layer 96, the insulatedlayer 56 around the resonator 12, and the oxidized layer 52 around theresonator 12 are removed by etching with dilute HF solution. By theprocess, the chamber 21 is formed around the resonator 12, and the gaparound the resonator 12 is kept.

On the other hand, by removing the oxide layer 96 formed between thefirst polysilicon layer 91 and the second polysilicon layer 97, the gap31 is formed between the first polysilicon layer 91 and the secondpolysilicon layer 97. The gap 31 extends from a first gap E1 between thefirst polysilicon layer 91 and the projection 98 to a second gap E2between the first polysilicon layer 91 and the second polysilicon layer97. The first gap E1 is communicated with the second gap E2. The diluteHF solution flows into the chamber 21 and flows from the chamber 21 viathe gap 31 to form the chamber 21.

By the manufacturing method of the resonant transducer, even if a littlewaste solution remains between the resonator 12 and the firstpolysilicon layer 91 and the meniscus force of the waste solution bendsthe resonator 12 widely toward the first polysilicon layer 91, theresonator 12 comes into contact with the end face 98 a of the projection98 projecting into the chamber 21. Therefore, it is preventable that theresonator 12 adheres to the first polysilicon layer 91.

Further, in the manufacturing method of the third embodiment, as across-section shape of the end face 98 a of the projection 98 issemicircle, a contact area of the projection 98 and the resonator 12 isvery small. Therefore, as the resonator 12 does not adhere to the endface 98 a of the projection 98, it is preventable that the resonator 12becomes deformed.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the scope of the present invention. Accordingly, theinvention is not to be considered as being limited by the foregoingdescription, and is only limited by the scope of the appended claims.

What is claimed is:
 1. A resonant transducer comprising: a siliconsingle crystal substrate; a silicon single crystal resonator disposedover the silicon single crystal substrate; a shell made of silicon,surrounding the resonator with a gap, and forming a chamber togetherwith the silicon single crystal substrate; an exciting module configuredto excite the resonator; a vibration detecting module configured todetect vibration of the resonator; a first layer disposed over thechamber, the first layer having a through-hole; a second layer disposedover the first layer; a third layer covering the first layer and thesecond layer; and a projection extending from the second layer towardthe resonator, the projection being spatially separated from theresonator, the projection being separated from the first layer by afirst gap, the second layer being separated from the first layer by asecond gap, the first gap is communicated with the second gap.
 2. Theresonant transducer according to claim 1, wherein the first layer, thesecond layer, and the third layer are made of any one of polysilicon,amorphous silicon, SiC, SiGe, and Ge.
 3. The resonant transduceraccording to claim 1, further comprising: a non-empty spacer forming thesecond gap, the non-empty spacer being integral with the second layer.4. The resonant transducer according to claim 1, wherein the projectionis disposed immediately above the resonator, and a distance between theprojection and the resonator is shorter than a distance between thefirst layer and the resonator.
 5. The resonant transducer according toclaim 1, wherein the through-hole is cuboid-shaped space extending alongthe longitudinal direction of the resonator, and a part of the gap isrectangular-section cylindrical between an outer surface of theprojection and an inner surface of the through-hole.
 6. The resonanttransducer according to claim 1, wherein a shape of the through-hole isoval or ellipse disposed along the longitudinal direction of theresonator, and a part of the gap is cylindrical between an outer surfaceof the projection and an inner surface of the through-hole.
 7. Theresonant transducer according to claim 1, wherein pressure in thechamber is less than atmospheric pressure.
 8. A manufacturing method ofa resonant transducer, the method comprising: (a) forming a resonator ina layered structure; (b) forming a first layer over the layeredstructure; (c) forming a sacrifice layer over the first layer; (d)forming a second layer having a projection over the sacrifice layer, theprojection extending from the second layer toward the resonator, and theprojection being spatially separated from the resonator; and (e) forminga gap between the first layer and the second layer by removing thesacrifice layer, the projection being separated from the first layer bya first gap, the second layer being separated from the first layer by asecond gap, the first gap is communicated with the second gap.
 9. Themanufacturing method of the resonant transducer according to claim 8,further comprising: forming dimples in the sacrifice layer in the step(c); and forming non-empty spacer integral with the second layeraccording to the shape of the dimples in the step (d), the non-emptyspacer forming the second gap.
 10. A multi-layer structure for aresonant transducer, the multi-layer structure comprising: a firstlayer; a second layer disposed over the first layer; and a projectionextending from the second layer toward the first layer, the projectionbeing separated from the first layer by a first gap, the second layerbeing separated from the first layer by a second gap, the first gap iscommunicated with the second gap.
 11. The multi-layer structure for theresonant transducer according to claim 10, wherein the first layer andthe second layer are made of any one of polysilicon, amorphous silicon,SiC, SiGe, and Ge.
 12. The multi-layer structure for the resonanttransducer according to claim 10, further comprising: a non-empty spacerforming the second gap, the non-empty spacer being integral with thesecond layer.